1. Field of the Invention
The present invention relates to a hollow multi-layer molding, and particularly to a hollow multi-layer molding that has excellent barrier properties (permeation-preventing properties) with respect to vehicle fuels (such as gasoline) and mixtures of hydrocarbons with oxygen-containing organic compounds (such as gasoline containing alcohol, i.e., gasohol), has excellent mechanical strength, such as impact strength, and is capable of being produced while allowing flash or the like to be recovered and reused (i.e., reclaimed) in an improved manner.
2. Description of the Related Art
Fuel tanks for vehicles have, through various approaches, been made of plastic materials by using blow molding techniques so as to achieve advantages such as improved safety upon impact, and improved freedom of shape design.
Properties required of plastic products to be used as fuel tanks include excellent properties for preventing permeation of fuel (gas barrier properties), and mechanical strength, such as impact strength, sufficient to be able to serve as fuel tanks.
Polyolefin, which is relatively cheap and which has excellent impact strength and similar mechanical strengths, is a material which may be suitably used to produce plastic fuel tanks. However, since polyolefin has poor gas barrier properties, various improvements have been proposed.
These proposals include a chemical treatment method in which the inner surface of a polyolefin tank is chemically treated with fluorine gas (F.sub.2) or sulfur trioxide gas (SO.sub.3), and a laminar method in which layers of modified polyamide comprising a combination of polyamide having relatively good barrier properties and polyolefin substituted by an alkylcarboxyl group, are formed in a continuous matrix phase of polyolefin.
However, these methods are not yet capable of assuring barrier properties sufficient for this use, and cannot be regarded as perfect since it may not be easy, with these methods, to cope with various environmental protection legal regulations which are expected to be more strict in the future. In addition, the laminar method has a problem in that the laminar state of modified polyamide tends to be unstable under some forming conditions, thereby involving the risk of reducing impact strength. The chemical treatment method has problems concerning the durability of treatment effect and a problem concerning environmental protection during the process.
Another proposal is made, for example, in Japanese Patent Publication No. 1-14049, which is a multi-layer formation method. In this method, a barrier layer made of a material having excellent gas barrier properties, such as a polyamide resin, a saponified derivative of an ethylene-vinyl acetate copolymer, or a thermoplastic polyester, is laminated to a polyolefin layer serving as the main layer with an adhesive layer of a modified polyolefin sandwiched between the main layer and the barrier layer.
This method provides products having much superior gas barrier properties. However, it is necessary, from an economical point of view, that a high proportion of reclaimed material, such as the multi-layer flash generated during molding, be mixed with virgin material (the proportion in weight of reclaimed material with respect to the total weight of the materials, however, generally being not more than 50% because reclaimed material may make product quality unstable if mixed at a proportion exceeding 50% by weight). Since such reclaiming decreases mechanical strength, such as impact strength, this strength needs to be further improved so as to be able to compensate for the decrease.